This invention relates to detectors of ionizing radiation such as X-ray and gamma radiation. The invention is concerned with improving multicell detectors which have various uses but are especially useful in X-ray computerized axial tomography systems.
In the computerized axial tomography process, a spatial distribution of X-ray photon intensities emerging from a body under examination are translated into analog electric signals which are processed in a manner that enables reconstructing the X-ray image and displaying it as a visible image. Background information on the process is given in an article by Gordon et al, "Image Reconstruction from Projections", Scientific American, October 1975, Vol. 233, No. 4.
Detectors used in computerized axial tomography must detect X-ray photons efficiently and with a high degree of spatial resolution. In some systems, the X-ray source is pulsed and the pulse repetition rate can be limited by the recovery time of the X-ray detectors. It is desirable, therefore, to use X-ray detectors which have fast recovery time, high sensitivity, and fine spatial resolution. In multicell detectors, it is also important for each cell to have identical and stable detecting characteristics.
In some tomography systems, the X-ray beam is fan-shaped and diverges as it exits from the examination subject whereupon the beam falls on the array of detector cells such that photon intensities over the leading front of the beam can be detected and resolved spatially. As the X-ray source and detector orbit around the examination subject jointly, the X-ray intensities across the diverging beam projected from the source are detected by the individual detector cells and corresponding analog electric signals are produced. The individual detector cells are arranged in a stack or array so that the X-ray photons distributed across the beam at any instant are detected simultaneously. The signals correspond with X-ray absorption along each ray path at the instant of detection. Additional sets of signals are obtained for the several angular positions of the orbiting detector and X-ray source. The discrete analog signals are converted to digital signals and processed in a computer which is controlled by a suitable algorithm to produce signals representative of the absorption by each small volume element in the examination subject through which the fan-shaped X-ray beam passes.
To get good spatial resolution, it is desirable to have the electrode plates which comprise each cell spaced closely and uniformly over the entire length of the detector. A detector in which advanced achievement of these results is disclosed in copending application Ser. No. 727,260, filed Sept. 27, 1976, now U.S. Pat. No. 4,075,527. This patent is assigned to the assignee of the present application. The detector in the cited application comprises a plurality of adjacent but slightly spaced apart electrode plates standing edgewise so as to define gas-filled gaps between them in which ionizing events, that is, the production of electron-ion pairs due to photon interaction with the gas, may take place. The plates are established at a uniform distance from each other by applying a heat curable viscous resin between them and between their insulator spacers as the plates are being stacked in a clamping die. The die squeezes the plates toward each other, and the resin yields to let the plates assume a uniform distance from each other. The assembly remains in the die during the heat curing process which effects solidification and bonding to mantain the plates at a fixed spacing. Although the dimensional tolerance of the spaces or gaps between the plates is good, it is still not as precise as experience has shown is necessary to get the high precision X-ray intensity data that is required for a reconstructed image of the highest resolution and definition.
Besides having the problems of dimensional tolerance, stability and difficult assembly, there is a problem of microphonics in arrays of parallel plate electrodes fabricated as described above. These electrodes must be made of thin metal and must operate in close proximity with a relatively large potential difference between them. Mechanical vibrations transmitted to the plates may, therefore, significantly vary the capacitance between electrodes and, thus, introduce microphonic current changes which are detected in the current sensing electronics and may cause errors in the X-ray intensity measurements. These spurious currents, which are in the picoampere range but, nevertheless, significant compared to the X-ray induced signal, have been measured in prior art detectors when they have voltage applied even though no X-ray photons were present.